Anisotropy is the variation of a physical property depending on the direction in which the property is measured. Anisotropy is a common phenomenon in various scientific fields, such as medical science, physics, and engineering. In the field of geophysics, anisotropy most often refers to seismic anisotropy. Seismic anisotropy is the dependence of seismic velocity upon angle, and can arise from intrinsic anisotropy of the rock itself, or from stress-induced anisotropy caused by a difference of directional stresses in formation layers. The application of seismic anisotropy has substantially improved the exploration of hydrocarbons, by modifying the velocity model from simple isotropic to more realistic anisotropic. Seismic anisotropy has played a role in applications such as long offset seismic data with greater angles of incidence (the angle-dependence of velocity is more evident), AVO (Amplitude versus offset) quantitative analysis, and anisotropic imaging (migration).